Protein misfolding diseases are known as neurodegenerative diseases caused by cytotoxicity due to abnormally aggregated proteins (misfolded proteins) (1). Alzheimer's disease is a representative protein misfolding disease, which causes deposition of the amyloid β-protein (Aβ) outside nerve cells in brain. Since initiation of therapy as early as possible leads to effective therapy of Alzheimer's disease, development of a method of early diagnosis of the disease is an important task in an aging society.
At present, NINCDS-ADRDA and DSM-IV, which are excellent for diagnosis of positivity of Alzheimer's disease, are employed as clinical diagnostic criteria, but with these criteria, the possibility that a patient is diagnosed as negative for the disease at an early stage of onset cannot be eliminated. Needless to say, it is impossible to give a diagnosis before onset.
It has been revealed that Alzheimer's disease is caused by accumulation of Aβ, and there have been reports wherein decrease in the Aβ42/Aβ40 ratio and increase in phosphorylated tau (p-tau) protein in the cerebrospinal fluid, and their combination p-tau/(Aβ42/Aβ40), were used as diagnostic indices. However, nerve cell death has already been progressed in the period wherein these values as diagnostic indices increase. Therefore, even with these indices, early diagnosis and prediction of onset of Alzheimer's disease are difficult.
On the other hand, recently, it has been revealed that neprilysin functions as a protease that degrades Aβ in brain (2), and it has been reported that early accumulation of Aβ oligomers in brain as well as cognitive dysfunction occur in neprilysin-deficient model mice for Alzheimer's disease (3 and 4).
Further, it has been reported that, in Alzheimer's disease patients, expression of neprilysin is decreased in hippocampus and temporal lobe gyri, wherein senile plaques are observed, so that the relationship between neprilysin and Alzheimer's disease has been suggested also in human (5).
On the other hand, in the fields of regenerative medicine and the like, a technology that enables conversion of a cell convenient as a biomaterial into a cell of a desired type is demanded, and recently, mouse and human iPS cells were established in succession. Yamanaka et al. introduced four genes, that is, Oct3/4, Sox2, Klf4 and c-Myc, into human skin-derived fibroblasts and succeeded in establishment of iPS cells (6 and 7). Since iPS cells that are obtained in such a process may be produced using cells derived from a patient to be treated and then allowed to differentiate into cells of various tissues, they are considered to enable reproduction of the diseased state in vitro. In fact, by the above method, iPS cells derived from a patient suffering from amyotrophic lateral sclerosis, which is a neurodegenerative disease, were produced, and the cells were successfully induced to differentiate into nerve cells (8).
So far, however, there has been no report in which nerve cells derived from iPS cells are used to analyze expression and functional change of a molecule involved in development of a protein misfolding disease, especially Alzheimer's disease, and the diseased state is reproduced, thereby carrying out early diagnosis of Alzheimer's disease.